1. Field of the Invention
The present invention relates to a scanning probe microscope and a method of measurement capable of measuring objects with a high aspect ratio or a wide-area scanning by approaching and separating a probe to and from a sample surface while continuing a servo control for the probe in a measurement by an atomic force microscope for example. More particularly the scanning probe microscope of the present invention is useful for enlarging the field of observation for evaluation of polishing in chemical mechanical polishing (CMP) to the millimeter class, which is highly needed in the field of the production of semiconductors.
2. Description of the Related Art
Representative examples of the scanning probe microscope are atomic force microscopes, magnetic force microscopes and tunnel microscopes. An atomic force microscope is utilized for measuring the topography of the surface of a semiconductor substrate, for example, as a sample. It utilizes the atomic force acting between a probe and the surface of the semiconductor substrate to measure or observe the fine topographic features of the surface. An actuator making the probe scan the surface of the substrate is generally a piezoelectric element. The amount of displacement given by the piezoelectric element is of the micrometer (μm) class, for example, and even at the largest is about 100 μm.
(1) First Related Art (Art Relating to Reproducibility of Measurement)
In measurement using the scanning probe microscope, the practice in recent years has been to measure a wide area by a wide scanning for example. In this case, the force acting on the probe in the horizontal direction arising due to the frictional force and attraction force lowers the precision and reproducibility of measurement.
More specifically, in the production of semiconductor devices, the miniaturization of the objects being produced has led to an increasing need to evaluate the surface flatness of the semiconductor substrate or the surface flatness of films deposited on a semiconductor substrate in the process of production of the semiconductor devices. In particular, in evaluation of the flatness in a flattening process using the CMP, measurement of the topographic features of a wide range from several mm (millimeters) to tens of mm with a nanometer resolution has been desired.
As a device enabling a large field of observation in such micro-measurement, in the conventional related art, instead of the scanning probe microscope, the device disclosed in Japanese Unexamined Patent Publication (Kokai) No.10-62158 has been proposed. The device disclosed in that patent publication is a surface roughness meter. In the section on the prior art, contact type and optical type surface roughness meters are described.
A contact type surface roughness meter brings a probe member into contact with the sample surface and scans the sample surface by the probe member to measure the topographic features of the sample surface. The movement of the probe member is detected by a differential transformer type detector. An optical type surface roughness meter detects the topographic features of the sample surface by an optical displacement detection system.
A surface roughness meter is configured to be able to observe the sample surface over a wide area by a mechanical structure. As this mechanical structure, for example, a motor driven type XY stage is used for the stage for making the sample move in the XY direction. The above probe member suffers from problems due to being structured to contact the sample surface such as deformation or destruction of the sample surface or low resolution due to the mechanical contact structure.
Therefore, the above patent publication proposes to use a cantilever such as used in the atomic force microscope instead of the probe member and thereby utilize the features of the atomic force microscope for the main part of the surface roughness meter.
In the scanning probe microscope of the related art configured for micrometer class measurement, when enlarging the scan range to the millimeter (mm) range, the distance of movement of the probe at the sample surface becomes longer. According to the normal method of measurement of the related art, during that movement, the probe moves in a state with its tip extremely close to the sample surface. Therefore, various problems such as the later mentioned frictional force or attraction force arise. The frictional force or attraction force lowers the precision and reproducibility of measurement.
Further, the above Japanese Unexamined Patent Publication (Kokai) No.10-62158 argues that the surface roughness meter proposed in the Publication can reduce the contact pressure applied to the sample surface compared with the conventional surface roughness meter for the reason of using the cantilever probe of the atomic force microscope. According to the surface roughness meter proposed in that publication, the scan stroke can be made larger. Even with this surface roughness meter, however, with just use of a cantilever of an atomic force microscope, possibility of reducing the reproducibility of measurement is high, so it is difficult to solve the above problem.
In wide-area measurement of the mm class scan range by the atomic force microscope etc., for quick measurement, generally, the probe is brought into contact with the sample surface at just the positions for sampling the measurement data and the height position of the probe with respect to the sample surface is held at a reference position. Locations other than the sampling positions are designated as distances for movement. In the movement distances, the probe is moved in the state retracted and separated from the sample surface. This configuration is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-5340, for example. In such a configuration, similarly, the problem arises of the reduction in the reproducibility of measurement due to the above problems when extending or approaching the probe to the sample surface for measurement tracking the surface.
Next the problem of the decline in the reproducibility of measurement will be explained with reference to FIG. 8 to FIG. 10. In FIG. 8 and FIG. 9, reference numeral 301 denotes a cantilever and 302 a probe attached to the tip of the cantilever 301. The cantilever 301 is drawn by two types of lines: thin solid lines and thick solid lines. The thin solid lines show the cantilever in the state before force is applied, while the thick solid lines show the cantilever in the state after force is applied. The tip of the probe 302 faces the surface of a not shown sample. The probe is approached close enough to the sample for the atomic force to act on both of them. On the back of the cantilever 301 is focused a laser beam 303 emitted from a laser light source (not shown) of a light lever type photo detection system. Further, the laser beam 303 reflected at the back of the cantilever 301 strikes for example a 4-division photodiode (not shown). At the cantilever 301, when the distance between the probe and sample is set to a predetermined value, a predetermined flex deformation occurs. When further force is applied to the probe 302 from the sample surface, the cantilever 301 undergoes further flex deformation as shown by the thick solid lines. The applied force determined the method of deformation.
FIG. 8 shows the desirable normal state of measurement. In FIG. 8, it is assumed that in the atomic force microscope only force vertical to the sample surface acts on the probe. In this state, only the displacement of the cantilever 301 due to the force acting on the sample surface in the vertical direction (Z-direction) is used as a signal source of a servo control system. When the scan of the sample surface by the probe is stopped, force does not act in the direction parallel to the sample surface due to friction, so the cantilever 301 flexes due to only the force in the vertical direction 304. As a result, the focal spot on the light receiving surface of the photodiode displaces in only the vertical direction as shown by reference numeral 305. Reference numeral 305 shows the position of the focal spot formed by the combination of a flex component and a torsion component.
On the other hand, if the probe 302 is made to scan the sample surface, frictional force occurs between the probe and sample. Due to the frictional force, force acts on the probe 302 in the direction (Y-direction) parallel to the sample due to the frictional force, so, as shown in FIG. 9, torsion occurs at the cantilever 301. Therefore, since the flex deformation and the torsion deformation components are combined, regardless of the fact that the force in the vertical direction does not change, the focal spot resulting from the reflection is made to displace in position as if there was a change due to the flex deformation. In such a case, the servo control system for the measurement drives the probe 302 in a direction to be pressed against the sample so as to try to return the focal spot on the light receiving surface of the 4-division photodiode to its original position. As a result, the problem arises that the force pressing against the probe 302 no longer becomes constant.
Further, the surface attraction force may be mentioned as a factor for force acting on the tip of the probe parallel to the sample surface. As shown in FIG. 10, in a normal atmosphere, the surface of the sample 307 is covered by adsorbed water 308. The thickness of the layer of this surface adsorbed water 308 is not constant. For example, when topographic features are formed on the sample surface, the top edge of a projection 309 becomes thinner in the thickness of the adsorbed water. The difference in thickness acts as a difference in force in a direction 310 parallel to the sample surface. Therefore, such an external disturbance acts at topographic features inherently posing a large load on the servo control system and the reproducibility of the measurement falls. Note that to facilitate the explanation, the explanation will be given about the case of scanning in a direction perpendicular to the longitudinal direction of the cantilever and the frictional force etc. causing torsional deformation. However, when scanning in any direction including a direction being consistent with the longitudinal direction of the cantilever, frictional force and other external disturbances cause the reproducibility of measurement to fall in the same way no matter what the direction.
When measuring a wide range by the atomic force microscope having a high nm class resolution, if a piezoelectric element is used as a scan actuator, the probe scan speed is about 10 μm/sec, so an extremely long time is taken. When scanning for example a range of 10 mm×10 mm and measuring the surface at 256×256 sampling positions, the measurement time becomes about 142 hours (512,000 seconds).
When using the above atomic force microscope to sample and measure a wide range of 10 mm×10 mm at 256×256 measurement points, it may be sufficient to obtain surface data once for every 40 μm. In accordance with the conventional measurement method, however, the probe was made to move so as to follow the topography of the sample surface even between measurement points for sample measurement, so the measurement efficiency was extremely poor. Therefore, to avoid this problem, the method of movement of the probe shown in the above Japanese Unexamined Patent Publication (Kokai) No. 2-5340 was proposed.
An example of the state of movement based on the configuration shown in Japanese Unexamined Patent Publication (Kokai) No. 2-5340 is shown in FIG. 11. In this figure, reference numeral 401 denotes a probe, 402 the path of movement of the probe 401, 403 a sampling position, 402a an approaching movement, 402b a separating movement, and 402c movement in the state separated from the sample surface 404. In such a configuration, since the probe 401 is made to move as shown by the movement path 402 utilizing a piezoelectric element, the speeds of movement of the approaching movement 402a and the separating movement 402b and the accuracy relating to the position become problems.
(2) Second Related Art (Art Relating to Approaching and Separating of Probe)
With the configuration of the scanning probe microscope disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-5340, the following important problem also arises.
The scanning probe microscope disclosed in this publication is a scanning tunnel microscope. The scanning tunnel microscope is provided with a servo control system. By approaching the probe to the sample surface and maintaining the tunnel current flowing between the probe and sample at a predetermined constant level, the distance between the probe and sample is held at a predetermined constant difference.
Further, since the probe is separated (or moved backward) at locations other than the sampling positions, it is necessary to suspend the control by the servo control system at the time of separating. The reason is that even if a backward signal is given to the actuator to control the height of the probe, if the above servo control is being continued, since the distance between the probe and sample will be held constant, the separating movement cannot be realized. Therefore, when moving the probe to the next sampling position and again approaching it to the sample surface, it is necessary to approach it slowly and carefully while producing an intermittently generated triangular drive signal and controlling the position in the height direction of the probe as shown by the time charts of FIG. 5 in the publication. Therefore, the problem arises that time is taken for the probe to approach to the sample. In addition, since the above approaching movement of the probe is necessary, the control of the probe movement becomes complicated.
(3) Third Related Art (Art Relating to Sample Surface with High Aspect Ratio)
Further, in the process of production of semiconductor devices in recent years, the increasing miniaturization of the objects being formed on the surface of the substrate has led to a need for a scanning probe microscope enabling observation of topographic features of a high aspect ratio. For example, holes formed for interlayer interconnections, called contact holes, have an aperture of 0.2 μm, but a depth of 1 μm. A scanning probe microscope able to measure the shape of holes with such high aspect ratios is therefore requested. The probe of the scanning probe microscope inevitably becomes a thin, long shape. For example, a probe with a high aspect ratio such as a diameter of 0.1 μm, a length of 1 μm, and a radius of the tip of 0.01 μm is used. If using such a probe for measurement by the conventional scan method of the related art, the problem arises of breakage or damage of the probe and remarkable tip wear of the probe.